1. Field of the Invention
The present invention relates to high temperature ceramic superconductors. More particularly, the present invention relates to methods of processing such high temperature ceramic superconductors.
2. Background of Related Art
Achievement of superconductivity at high temperature, i.e. temperatures greater than that of liquid nitrogen, is of tremendous technological importance. The number of potential applications for such high temperature superconductors are clearly enormous. Therefore, the announcement of superconductivity at approximately 30K by Bednorz and Muller, Z. Phys. B. 64, 189 (1986) in certain lanthanum barium cupric oxide ceramic materials has generated unprecedented excitement and efforts in the scientific, technological and business communities. Following the announcement of the work of Bednorz and Muller, these efforts have generated several significant and rapid increases in the superconductivity onset temperatures. Chu and others have reported superconductivity over 90.degree. K in yttrium barium cupric oxide ceramic compounds. M. K. Wu, J. R. Ashburn, C. J. Torng, P. H. Hor, R. L. Meng, L. Gao, C. J. Huang, Y. Q. Wang, and C. W. Chu, Phys. Rev. Lett. 58, 908 (1987), C. W. Chu, P. H. Hor, R. L. Meng, L. Gao, Z. J. Huang and Y. Q. Wang, Phys. Rev. Lett. 58, 408 (1987). More recently, superconductivity at 155K with evidence of superconductivity onset above room temperature has been reported. S. R. Ovshinsky, R. T. Young, D. D. Allred, G. DeMaggio, G. A. Van der Leeden, Phys. Rev. Lett. 58, 2579 (1987). The latter result involved a multiphase ceramic material of yttrium, barium, copper, fluorine and oxygen.
Despite the tremendous scientific efforts devoted to the study of the new high temperature ceramic superconductors, and despite the significant and rapid series of increases in the superconductivity onset temperature, several key problems have remained without satisfactory solution. Two of the most significant of these problems relate to the lack of phase purity and crystal purity in the structure of the ceramic superconductors.
The first of these problems relates to the presence of two or more distinct, structurally different, phases present in the bulk samples of the high temperature ceramic superconductor material. This first type of phase purity problem is illustrated in a photomicrograph of a polished section of a Y-Ba-Cu-O mixed phase 93K ceramic superconductor as shown in FIG. 1 of R. J. Hemley, H. K. Mao, Phys. Rev. Lett. 58, 2340 (1987). Multiphase structure has also been reported in the Y-Ba-Cu-F-O high temperature superconductor compounds of Ovshinsky et al. The differing phases of the ceramic superconductors may be characterized by different crystal structures as well as variations in oxygen site vacancies in the lattice structure, and other differences. The principal problem associated with the multiphase nature of the ceramic superconductors derives from the fact that the differing phases will typically have differing superconducting onset temperatures. For example, in the Y-Ba-Cu-F-O ceramic superconductors reported in Ovshinsky et al., the evidence of superconductivity above room temperature suggests one of the phases in the sample has a superconductivity transition temperature above room temperature, whereas the superconductivity transition temperature of the lowest temperature phase is approximately 155K. Clearly achievement of bulk high temperature superconductivity requires that the desired phase with the highest transition temperature comprise substantially all of the ceramic superconductor material.
The second serious problem relates to the crystal structure alignment of the high temperature ceramic superconductors. This problem derives from the acknowledged two-dimensional nature of the superconductivity in these ceramic superconductors. For example, the two-dimensional nature of the properties of the lanthanum barium cupric oxide ceramic superconductors is discussed in Jaejun Yu, A. J. Freeman, and J-H Xu, Phys. Rev. Lett. 58, 1035 (1987). Further evidence of the two-dimensional nature of the superconductivity effect in the ceramic superconductors is provided in P. H. Hor, R. L. Meng, Y. Q. Wang, L. Gao, C. J. Huang, J. Bechtold, K. Forster and C. W. Chu, Phys. Rev. Lett. 58, 1891 (1987). In FIG. 1, the two-dimensional structure of certain lanthanum barium cupric oxide superconductors is shown schematically (taken from Hor, et al., Phys. Rev. Lett. 58, 1892). The two-dimensional or planar nature of the superconductivity in the high temperature ceramic superconductors has associated with it a high degree of anisotropy in the properties of the superconductors including the current carrying properties, magnetic properties, and others. Due to this anisotropy, a bulk sample with randomly oriented grains of even a single phase ceramic superconductor will have greatly reduced current carrying capacity over a sample with an ideal pure crystal structure or grains with an aligned planar structure. This is due to the necessity of the current to percolate through the randomly oriented planar structures in the bulk material. This source of the current limitations in bulk ceramic superconductors is supported by the announcement by IBM of greatly increased current carrying capacity in meticulously grown single crystal films of ceramic superconductors.
The tremendous technical significance of the above-noted problems is matched, if not equalled, by the commercial significance of the aforementioned problems. See, for example, Wall Street Journal, Jul. 9, 1987, p. 1, col. 1. This article also reports attempts to solve the two-dimensional current carrying problem, including that of IBM, all of which are characterized by meticulous efforts to grow single crystal structures.
Accordingly, at present a great need exists for a method for providing bulk ceramic superconductors with improved phase purity and improved crystal structure alignment.